perf(core,viewer): stop rebuilding wall geometry every frame

[toycenterboss-bot]

What does this PR do?

Wall geometry, mitre joints and slab support were recomputed inside the render
loop, so every frame paid for work whose inputs had not changed. On an imported
floor with 1089 walls the first level build took 59 s and the editor was
unusable until it finished. This PR caches the derived results and invalidates
them only when the walls, the level or the openings actually change: mitre
solving keeps a per-level result keyed by the exact wall fields the miters
depend on, slab support holds its effective slab/wall records by identity
instead of re-deriving them once per wall, and the rendered-polygon lookup
memoises against those same arrays. The same level now builds in 0.01 s.

No behaviour changes — the geometry produced is identical, it is simply produced
once instead of once per frame.

Related to #492 — the per-frame recompute is one of the paths that makes wall
interaction stutter on large scenes. Measurements and method are in
a comment on that issue.

Scene	Wall nodes	First level build
imported floor plan	1089	59 s → 0.01 s

The second commit clears the new level-miter cache when WallSystem unmounts —
it lives at module scope, so without that a remount or a second project in the
same tab keeps the previous level's wall arrays reachable. Editor teardown
already resets the other shared singletons; this makes the cache follow the same
rule, and the rule is now written down in wiki/architecture/systems.md.

How to test

1. bun dev and open a scene with a few hundred walls (any floor plan import will do).
2. Watch the time between opening the scene and the first painted frame — it should
   be immediate rather than a multi-second freeze.
3. Orbit the camera and confirm wall joints, openings and slab-supported wall tops
   look exactly as they did before.
4. Move a wall with the move tool and confirm its neighbours re-mitre immediately —
   the cache must invalidate, not go stale.
5. Navigate out of the editor and back in, then open a different project in the same
   tab — the miter cache must not carry entries from the previous level.
6. bun test — wall-mitering, slab-support and spatial-grid suites cover the
   invalidation paths.

Screenshots / screen recording

N/A — no visual change. The output geometry is identical; only the time to produce
it changes. Happy to add a before/after screen recording of the load freeze if that
would help review.

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch

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